1. Field of the Invention
This invention relates to communication networks. More specifically, the present invention relates to communication over optical networks.
2. Description of the Related Art
Current networks must satisfy consumer demand for more bandwidth and a convergence of voice and data traffic. The increased demand of bandwidth by consumers combines with improved high bandwidth capacity of core networks to make edge networks a bottleneck despite the capacity of optical networks.
Multiplexing is used to deliver a variety of traffic over a single high speed broadband line. An optical standard such as Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) in conjunction with a multiplexing scheme is used to deliver various rates of traffic over a single high speed optical fiber. SONET/SDH is a transmission standard for optical networks which corresponds to the physical layer of the open standards institutes (OSI) network model. One of the protection schemes for SONET/SDH involves automatic protection switching (APS) in a bi-directional line switched ring (BLSR) architecture. BLSR utilizes linear switching to implement APS.
FIGS. 1a-1e are diagrams illustrating an example of traffic flow in a Bi-Directional Line Switched Ring (BLSR) while there is and is not a failure in the ring. FIG. 1a (Prior Art) is a diagram of exemplary traffic flow on a BLSR while there is not a failure. Although a BLSR has a working channel and a protection channel for traffic flowing East and West, only one working channel and its protection channel (which traverse the ring in the opposite direction) are shown in FIGS. 1a and 1b. In FIG. 1a, a stream of traffic 113 is received from a source external to the ring at node 101. Node 101 transmits this traffic 113 over its East span 115 on a working channel 119 to a node 103. Node 103 transmits the traffic 113 over its East span 117 in a working channel 121 to node 105. The stream of TDM traffic 113 exits the ring at node 105 to a destination external to the ring. Although extra traffic may be flowing in the protection channels of the ring, only the stream of TDM traffic 113 is shown for simplicity.
FIG. 1b (Prior Art) is a diagram of exemplary traffic flow on the BLSR while there is a failure. In FIG. 1b, the node 103's East span 117 has failed (e.g. severed lines). The stream of TDM traffic 113 is protection switched at node 103. Node 103 informs the other nodes in the ring of the failure. The stream of TDM traffic 113 is transmitted back to node 101 from node 103 in the protection channel 110 of node 103's West span 115. The stream of TDM traffic 113 continues around the ring to node 105 along a protection path. The protection path includes the protection channels 114, 120, 128, and 120 carrying traffic between nodes 101 and 107, 107 and 109, 109 and 111, and 111 and 105 respectively.
FIG. 1c (Prior Art) is a diagram of exemplary traffic flow on the BLSR while there is not a failure. In FIG. 1c, transmit working and protection channels 137, 110 and receiving working and protection channels 119, 139 of node 103's West span 115 are shown. Similarly, transmit working and protection channels 121, 135 and receiving working and protection channels 141, 143 of node 103's East span 117 are shown. The transmit working channel 137 and the receiving protection channel 139 of node 103's West span 115 are not shown in FIGS. 1a and 1b for simplicity. The transmit protection channel 135 and the receiving working channel 141 of node 103's East span 117 are also not shown in FIGS. 1a and 1b for simplicity. A stream of working TDM traffic 104 is transmitted in the transmit working channel 137 from node 103 to node 101. Another stream of working TDM traffic 113 is received in the receiving working channel 119 and transmitted to node 105 in the transmit working channel 121 while there is not a failure. The receive working channel 141 carries TDM traffic not shown in the figure.
FIG. 1d (Prior Art) is a diagram of exemplary traffic flow on the BLSR while there is a failure. In FIG. 1d, the stream of working TDM traffic 104 continues to be transmitted in the transmit working channel 137. The stream of TDM traffic 113 is protection switched to the transmit protection channel 110 while there is a failure.
The ring described in FIGS. 1a-1d can be a 2 fiber or 4 fiber BLSR. The channels described in FIGS. 1a-1d are logical channels which may reside on different optical fibers depending on the ring architecture. A ring switch, which is a protection switch that occurs in both 2 fiber and 4 fiber BLSRs, is illustrated in FIGS. 1c-1d. 
FIG. 1e (Prior Art) is a diagram illustrating a span switch for a 4 fiber BLSR while the transmit working channel 121 of FIGS. 1c-1d fails. In FIG. 1e, the transmit working channel 121 of node 103 fails. In a 4 fiber optical ring, the failure is detected and the stream of TDM traffic 113 is span switched to the transmit protection channel 135. A span switch is a protection switch which occurs in a 4 fiber BLSR. Physically, the East span 117 is 2 fibers. The transmit working channel 121 exists on one fiber and the transmit protection channel 135 exists on a separate fiber. The failure of the working channel 121 is a failure of the first fiber. In this example, the two fibers 121 and 133 are in separate conduits. Since the fibers run in separate conduits, a failure caused by severing will not affect both fibers. The stream of TDM traffic 113 is switched from being transmitted over the first fiber to being transmitted over the second fiber.
High speed optical rings offer large amounts of bandwidth, but the protection scheme utilizes at most 50% of that bandwidth. This 50% of maximum possible total bandwidth for a protection channel often goes unused while there is not a failure. It is often unused because traffic transmitted in the protection channel would be preempted by the working TDM traffic while a failure occurs.
FIGS. 2a and 2b are diagrams illustrating the use of a protection channel to carry extra time division multiplexed (TDM) traffic while there is and is not a failure . FIG. 2a (Prior Art) is a diagram illustrating the use of a protection channel to carry extra TDM traffic while there is not a failure. In FIG. 2a, a West span 201 is divided into a working channel 205 and a protection channel 207. The working channel 205 carries TDM traffic 209 and the protection channel 207 carries extra TDM traffic 211. An East span 203 is also divided into a working channel 204 and a protection channel 206. The working channel 204 of the East span 203 carries TI)M traffic 213 and the protection channel 206 carries extra TDM traffic 215.
FIG. 2b (Prior Art) illustrates preemption of extra TDM traffic while there is a failure. In FIG. 2b, the East span 203 has failed. The working TDM traffic 213 is protection switched into the protection channel 207 of the West span 201. The protection switched working TDM traffic 213 preempts the extra TDM traffic 211 which was previously carried in the protection channel 207 of the West span 201. The extra TDM traffic 215 previously transmitted over the protection channel 207 of the East span 203 is not protected and is therefore completely lost upon the failure. The extra TDM traffic is problematic to sell to customers because it is preemptable and unprotected. A consumer could purchase the extra traffic service from two network owners or providers and alternate between the two upon failures. While the above is true for a 2 fiber BLSR, the impact to extra TDM traffic in a 4 fiber BLSR depends on the type of failure. In particular, while a ring switch in 4 fiber BLSR operates in a similar manner as described above, a span switch in a 4 fiber BLSR does not impact the extra TDM traffic transmitted on the non-failing spans.
An alternative to unprotected preemptable traffic in a protection channel is to provide a non-preemptable unprotected traffic (NUT) channel. A NUT channel allows for an implementation that runs a unidirectional path switched ring (UPSR) over a BLSR. Other examples include ATM 1+1 protection schemes which can traverse over the NUT channel. Thus, a NUT channel is used to provide two circuit level protection schemes.